Ancon sheep are a grouping of sheep with long bodies and very short legs, with the fore-legs being crooked. The term is generally applied to a line of sheep bred from a single affected Massachusetts lamb born in 1791. The breed was artificially selected and maintained for its desirable inability to jump over fences, however it was allowed to go extinct in 1876 when it was no longer required.

The name "ancon" has also been applied to other strains of sheep arising from individuals with the same phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

, such as a Norwegian

Norway

Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

 stock bred from a single individual born in 1919, and a Texan

Texas

Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

 stock bred from a single individual born in 1962. These lineages were also allowed to go extinct after scientists no longer needed them for genetic research.

Excavations in Leicester

Leicester

Leicester is a city and unitary authority in the East Midlands of England, and the county town of Leicestershire. The city lies on the River Soar and at the edge of the National Forest...

, UK have also revealed metacarpals, metatarsals, and phalanges characteristic of ancon sheep that date to approximately AD 1500, thereby demonstrating that the phenotype has arisen independently at least four times.

The unique features of ancon sheep are caused by a recessive dwarf

Dwarfism

Dwarfism is short stature resulting from a medical condition. It is sometimes defined as an adult height of less than 4 feet 10 inches  , although this definition is problematic because short stature in itself is not a disorder....

 mutation, which often results in crippling. The specific effect of the mutation is to cause chondrodystrophy

Chondrodystrophy

Chondrodystrophy refers to a skeletal disorder caused by one of myriad genetic mutations that can affect the development of cartilage....

.